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Risks Not Limited to Chemicals
Clearly, another reason why just banning all carcinogens known to man would not solve the problem is that environmental risks of cancer are not limited to chemicals. A 1995 study in the American Journal of Epidemiology found that dogs living in houses with high magnetic fields, as measured by the current configuration wire code, were nearly seven times more likely to develop lymphosarcoma. This included both cables above and below ground. Interestingly, Moore states that these findings have never been shown in humans. Another non-chemical risk factor for cancer in dogs is the failure of the owner to have the dog spayed. This is one of the most common and well-documented risk factors for cancer in female dogs. Says Moore, “Female dogs who are intact are much more likely to develop mammary cancer than those who are spayed. Spaying before the first heat cycle reduces the risk to almost zero.” This risk steadily increases with each heat, up to about 6 to 8 percent risk of mammary cancer after the dog has gone through two to three cycles. Once the dog hits about two and a half years of age, 40 percent will develop mammary cancer. Says Moore, “This is a disease we can practically eliminate by spaying dogs early.”

Additionally, diet can further reduce the risk of mammary cancer. One research study looked at the diet of dogs the year before they had surgery for mammary cancer. For dogs on a low-fat diet, the level of protein (measured on a dry-matter basis, not based on the crude analysis number reported on the bag) was strongly predictive for how long they would live. Protein greater than 27 percent on a dry-matter basis correlated with survival past three years of age. Those dogs on a low-fat diet with less than 23 percent protein survived less than six months. Protein levels made no difference or had no effect if the dog was on a high-fat diet. In addition to diet composition, the researchers also found those dogs that were overweight at one year of age were three times as likely to develop mammary cancer.

More Studies May Provide Better Answers
So should you just avoid all possible carcinogens?

“That’s fine if you don’t drink or eat,” says Glickman, “but they are everywhere.” Even environmental factors as ubiquitous as sunlight can increase risk of cancer, especially in light-skinned pets. It’s a matter of exposure levels combined with genetics.

How high does the risk have to be to be important? Glickman recommends that we consider how likely the animal is to be exposed. A 20 percent increased risk may be important for dogs who are exposed a lot, whereas a twofold increased risk is less worrisome if the animal will not receive much exposure. Thus, for Scottish Terriers, a breed in which it is calculated that 75 percent of bladder cancers are related to chemical exposure, it is best to avoid phenoxyherbicides and insecticides, whereas a moderate exposure is much less likely to cause a problem for other dogs.

In defining “a lot” and “not much,” Glickman notes that “this speaks to the concept of attributable risk, which is the proportion of a specific type of cancer that can be attributed to a specific exposure such as insecticides. Attributable risk is a function of how many individuals in a population are exposed to insecticides plus the risk of cancer associated with insecticides. Thus, a chemical associated with a small increased risk of bladder cancer is important if a high proportion of dogs are exposed. In contrast, a chemical like asbestos is associated with a very high risk of mesothelioma, but very few dogs are exposed, and the attributable risk is thought to be low in people (about 5 percent of all lung cancers). In contrast, the attributable risk for lung cancer (90 percent) associated with smoking is very high even though the risk of lung cancer associated with smoking is relatively low, since so many people smoke.”